In conventional fuel injectors there is a single actuator for a spill control valve controlled by means of a drive circuit. A drive circuit for controlling a four cylinder internal combustion engine having four fuel injectors is described in PCT application WO96/27198. The actuator for each fuel injector valve has a winding through which a current is passed to actuate the valve. A first controllable switch (commonly referred to as the high side switch) is connected in series between one end of the winding forming part of each actuator and a first terminal of the drive circuit connected to the positive terminal of a DC supply. The high side switch is therefore connected to one end of each of the four actuators. Additionally, a diode is connected in the path between the DC supply and the high side switch.
The other end of each winding is connected in series to a second controllable switch (commonly referred to the low side switch), which in turn is connected to the negative terminal of the DC supply via a resistor. The current flowing through each winding can be determined by means of this resistor. In addition, the anode connection of a second diode, one associated with each winding, is in connection with the end of the winding connected to the respective low side switch. The cathode connection of each second diode is in connection with the negative terminal of the DC supply via a capacitor. Thus, depending on whether the low side switch of the associated winding is switched on or off, the winding of each valve connects to the negative terminal of the battery either via a capacitor or by means of the resistor.
When it is required to actuate one of the valves, the current in the associated winding is allowed to rise to a high value and is then allowed to fall to a lower value after which it is maintained for a period at a mean level by subjecting the high side switch to a modulated pulse, thereby serving to regulate the current flow through the windings, until such time as the current is turned off to de-actuate the valve.
In more recently developed internal combustion engines, dual-valve fuel injectors are to be employed, each fuel injector having a spill valve and a nozzle control valve requiring independent control by an associated actuator. Thus, there is a requirement for a drive circuit for controlling an assembly of fuel injectors each having two actuators. For this purpose it would be possible to employ the drive circuit for a conventional single-actuator fuel injector assembly, as hereinbefore described. However, in order to do so, a duplicate of each circuit component is required for the additional actuator in each fuel injector. This requires a drive circuit of considerable complexity. In particular, four wire connections are needed to each fuel injector from the DC supply, two for each winding of each actuator. This is particularly difficult to accommodate within a single fuel injector body.
It is an object of the present invention to provide a drive circuit for a dual-actuator fuel injector having reduced complexity.